Influence of carbon nanotubes on the plasma electrolytic oxidation process of aluminum under “soft” sparking conditions

Abstract

Plasma electrolytic oxidation of aluminum (PEO) was performed with dispersed multi-walled carbon nanotubes (MWCNTs) into the electrolyte. The effect of the carbon nanotubes concentration, as well as that of the electrical processing conditions, on the PEO process was investigated. From the point of view of the electrical response of the process, results show that the “soft” sparking regime appears earlier in presence of MWCNTs. The process time at which the transition to the “soft” regime occurs decreases linearly with the increase in the MWCNTs concentration. Over a concentration threshold, depending on the applied pulse bipolar current waveform, the PEO process is inhibited. The dispersion of MWCNTs in the electrolyte results in thicker but more porous alumina layers. More specifically, the proportion of the α-alumina phase relative to the γ-alumina phase decreases with the increase in the MWCNTs concentration. Results are explained by considering the increased coating growth kinetics and lower heat accumulation from MWCNTs addition. Finally, the present study gives new insights into the right management of the PEO process under “soft” sparking regime with dispersed carbon-based nanoparticles in the electrolyte.